Adolescents who engage in non-opioid substance misuse and/or experience mental health symptoms are at greater risk of misusing opioids and/or developing opioid use disorder. Adolescence is a critical developmental period to both prevent the initiation of opioid misuse and target mental health. To date, there are no digital health games targeting both conditions. We describe the protocol for a randomized controlled trial designed to assess the efficacy of an original digital health game, PlaySmart. Five hundred and thirty-two adolescents aged 16–19 years old, who are at greater risk for initiating opioid misuse are recruited from 10 Connecticut school-based health sites. Participants are randomized to PlaySmart or a set of time/attention control videogames. Randomization was stratified by sex at birth and school grade. Participants play their assigned game or games for up to six weeks (300 minutes) and complete assessment questions over a 12-month period (baseline, post-gameplay, 3, 6, and 12 months). The primary outcome is perception of risk of harm of opioid misuse at 3 months. Secondary outcome measures specific to opioid misuse include intentions, self-efficacy, attitudes, knowledge, and perceived norms. Mental health outcomes include measures of depression (Patient Health Questionnaire-8), anxiety (Generalized Anxiety Disorder-7), help-seeking behaviors, stigma, measures of self-regulation, self-efficacy to seek professional help for mental health, and knowledge around coping skills. PlaySmart has the potential to significantly reduce the risk of initiation of opioid misuse, improve mental health outcomes, and given its high levels of engagement and accessibility, holds the promise for extensive reach, scale, and impact for adolescents.